Piston Inline Spark-ignition Engine

In a gasoline (Otto, SI) engine, a mixture of fuel and air is drawn into the combustion chamber at the top of the cylinder during the downward stroke of the piston.   The next upward piston stroke compresses the mixture to about one-tenth of its original volume, and as the piston reaches its upper limit of travel, the mixture is ignited by an electric spark from the spark plug.   The fuel mixture burns and results in its expansion, which pushes the piston back down the cylinder.   SI engines give better mileage (more miles to the gallon), but diesel engines are more economical (more miles per dollar) and durable.   Autos, motorcycles, and small airplanes usually use SI engines, whereas trucks, taxis, buses, tractors, and special machinery usually use CI (diesel) engines. ^{How 704}

For automobile gasoline engines, see car engines.